Humidity is a measure of the water vapor content of the air. The amount of water vapor in the air depends on the controls over evaporation discussed earlier. There are several ways in which a meteorologist can express the humidity of the air. Each humidity measure is controlled to some degree by air temperature.
Absolute humidity is the weight of water vapor per unit volume of air, usually measured in units of grams of water vapor per cubic meter of air. Absolute humidity is not often used to express the moisture content of air because it is sensitive to changes in both the temperature of the air and atmospheric pressure. For instance, let's say that a 1 cubic meter parcel of air at the surface has 2 grams of water in it. Now lift the parcel of air upwards into the atmosphere. As the air rises upward the decrease in atmospheric pressure on the parcel allows it to expand outward occupying more space. Let's say that the parcel doubles in size as a result of uplift. Before rising, the absolute humidity was 2 gm/m3. As the air doubles in volume the new absolute humidity is 1 gm/ m3. In actuality the parcel still has the same weight of water in it, 2 grams. But given the way absolute humidity is calculated it appears the amount of water in the air has decreased.
Instead of absolute humidity, we use a measure that is not sensitive to volume changes in the air. Specific humidity is measured as the weight of water vapor in the air per unit weight of air, which includes the weight of water vapor. The units of measurement are grams of water vapor per kilogram of air. Given that weight is not significantly influenced by temperature or atmospheric pressure, specific humidity is much more useful as a measure of humidity. Another measure very similar to specific humidity is the mixing ratio. The mixing ratio is the weight of water vapor per unit weight of dry air. Because the atmosphere is made up of so little moisture by volume, the mixing ratio is virtually the same as the specific humidity.
Humidity is not only measured as a weight, but also by the pressure it creates. Vapor pressure is the partial pressure created by water vapor. Vapor pressure, like atmospheric pressure, is measured in millibars and is relatively insensitive to volumetric expansion or temperature. The saturation vapor pressure is simply the pressure that water vapor creates when the air is fully saturated.